It's not because you're tired. Well, it is. But it's also very much to do with the company you keep.
New research from Baylor University in Texas has found there's a correlation between empathy and how much yawning you do.
A survey of 135 people measured psychopathic traits to see how well participants empathised with others. Then the participants were shown videos of people laughing, yawning, and with expressionless faces. The higher people scored in the empathy stakes, the more likely they were to yawn along too.
There's also a link between how well you know someone and how likely you are to yawn, researchers found.
So if you're one of those people who suffer from contagious yawning, you're more likely to do it when it's someone you know well than an acquaintance or stranger.